10 



CaJtha palustris L. 



Cavity-formation in this plant is very similar to that in the leaves 

 of Allium cepa. In the young stem before half elongated the cortex 

 and pith are composed of plates of cells separated by large schizo- 

 genous canals giving in cross-section a net-like appearance. As growth 

 progresses, the net work in the pith suffers many rents, the separating 

 cells living for some time after they have been torn apart. By this 

 process of separation some cells are left with but small connection 

 with their neighbors and such are the first to die and collapse. In 

 this partly schizogenous, partly lysigenous way the cavity grows till 

 it arrives within a few cells of the bundles. A central cavity is 

 present about the time that elongation ceases. 



Gypsum casts were applied to 4 stalks, in 2 cases before any 

 dead cells were present within the segment encased, and in the other 

 2 before a general cavity was present, the former being thus the less 

 developed. At this time also there was no mechanical tissue present. 



The first plant was examined 20 days after the cast was applied 

 and was one of the 2 in which certainly no dead cells were present 

 in the pith at the time of application. This plant as well as the 

 other 3 grew about as well as its normal neighbors. Elongation was 

 not complete till after casting, and hence the swelling above the cast 

 was gradual, not sudden. 



Above and below the cast a large cavity was found in the pith, 

 its size above the gypsum being two thirds the diameter of the stem. 

 Within the cast was no cavity, but there were a few dead cells crushed 

 together in the centre, closely pressed by the surrounding living tissue. 

 The mass of living pith cells within the cast was greater than that 

 above and below. The preexisting large intercellular spaces in pith 

 and cortex had been nearly closed up. The bundles within and out- 

 side the cast were about equally well developed. Above and below 

 the cast the bundle-sheaths were beginning to show thick walls; within 

 the cast all cells were thin-walled. There was no evidence that the 

 bundles had been crowded toward the centre; the great spaces in 

 the cortex had afforded room for their growth. 



The other 3 plants were examined after 52 days. They had all 

 grown well and had produced ripe seeds. 



In the one in which there was assumed to be no dead cells at 

 the time the stem was surrounded with gypsum, the changes between 

 normal and abnormal growth had gone on in the same direction as 



